New second paragraph:

Until today's release, only MasterCard would work with the NFC payment system, leaving Visa and Discover cardholders with no way to use it. The phone-based "touch-to-pay" application should now work with all four.

New last paragraph:

UPDATE: American Express has since stated publicly that, although negotiations are ongoing, it does not yet officially support Google Wallet. Users can link their Amex cards to the system for the moment, but reports indicate that the credit card issuer can turn that functionality off at will.